1. Field of the Invention
This invention relates to a process for producing a novel semi-synthetic blood substitute and the novel semi-synthetic blood substitute resulting therefrom. The novel semi-synthetic blood substitute is a hemoglobin preparation characterized by its purity, its exceptionally low levels of endotoxin, the absence of non-hemoglobin proteins, and its molecular cross-linking profile. The semi-synthetic blood substitute has no toxic activity when used in a substitute fashion and possesses the property of reversibly binding gaseous ligands such as oxygen and is useful for transporting and supplying oxygen to vital tissues and organs. Additionally, the blood substitute serves as a blood plasma expander for management of disease and for maintaining circulatory integrity. A further aspect of the invention is the preursor or intermediate, the substantially pure, phospholipid-free, endotoxin-free hemoglobin solution in uncross-linked form.
2. Description of Background Materials
Complex multicellular organisms are equipped with specialized tissues which are concerned with the processes of nutrition and excretion. It is the primary function of blood to provide a link between various organs and cells of the body. Blood, red cells, plasma and other components maintain a constant cellular environment by circulating through every tissue and continuously delivering nutrients to the tissues and removing waste products and various tissues which are concerned with the tissue secretions from them. PHYSIOLOGY, Third Edition, Edited by Edward E. Selkurt, Page 223 (1971). Blood is a viscous fluid composed of cells and plasma. More than 99% of the cells are red blood cells. The major function of red blood cells is to transport hemoglobin, which in turn carries oxygen from the lungs to the tissues and CO.sub.2 from the tissues to the lungs. Normal red blood cells contain approximately 34 grams of hemoglobin per 100 ml of cells. Each gram of hemoglobin is capable of combining with approximately 1.33 ml of oxygen. See Guyton, A. C., BASIC HUMAN PHYSIOLOGY: NORMAL FUNCTION IN MECHANISMS OF DISEASE, Pages 84-85( 1971).
Because of the critical and ongoing need for a therapeutic agent useful as a blood substitute for carrying and supplying oxygen and as a blood plasma expander, intense research efforts have been directed to the development of an adequate blood substitute. The need for a blood substitute exists for replacing blood lost by acute hemorrhage, blood losses occuring during surgical operations, resuscitation procedures after acidental blood loss, and the like. Further, as a plasma expander, a blood substitute serves as a therapeutic to treat volume deficiency shock, as an alleviant in anaphylactic and allergic shock, and for replacing plasma lost after burns and as a result of severe diarrhea.
Hemoglobin in solution has the capability to transport oxygen and, theoretically, could be used as a substitute for red blood cells. Because hemoglobin solutions are oncotically active, these solutions also expand plasma volume, thereby providing a function as a plasma expander as well. Thus the ability to be oncotically active and transport oxygen suggests that hemoglobin solutions would be desirable for a resuscitation fluid where rapid initial treatment of hypovolemia and tissue hypoxia is required. However, in order to function as an adequate resuscitation fluid, hemoglobin solutions must be capable of maintaining tissue oxygenation for specified periods of time.
Hemoglobin present in the blood of mammals has a fundamental property in solution of reversible oxygenation. In its natural form, mammalian hemoglobin is a conjugated, non-crosslinked protein having a molecular weight of approximately 68,000 and structurally comprised of two pairs of sub-units. Each sub-unit contains a heme group and a polypeptide chain, called globin. In mammals, hemoglobin is present in erythrocytes, along with stroma which consists of proteins, phospholipids and cholesterol. See CLINICAL HEMATOLOGY, By Wintrobe, 6 Ed. Pages 138-199, (1967).
The reversible binding of oxygen requires the interaction between four chains of hemoglobin (tetrameric hemaglobin) which results from the ability of the protein to exist as two different quarternary structures (relaxed and tense) that have different oxygen affinities (Perutz, M. F. , Prog. Clin. Biol. Res. 1: 3 (1975)). The two different oxygen affinities permit hemoglobin to on-load oxygen when 1) the oxygen tension is high (approximately 100 mm Hg pO.sub.2) and to off-load oxygen when the oxygen tension is low (approximately 40 mm Hg pO.sub.2) and give rise to a characteristic sigmoidal shape to the oxygen-hemoglobin dissociation curve. It is now known that the tense state of some hemoglobin in red cells is stabilized by the presence of organic phosphates such as 2,3-diphosphoglycerate (2,3-DPG), with the tense state of hemoglobin in solution not stabilized due to the absence of 2,3-DPG. Accordingly, hemoglobin in solution has a lower P.sub.50 than hemoglobin in its natural form (Arnone, A., Nature 237: 146 (1972).
Aqueous hemoglobin exists in equilibrium between the tetrameric (MW 68,000) and dimeric (MW 34,000) forms (Bunn, H. F. et al., Trans. Assn. Am. Physicians 81:187 (1968)). The dimers are excreted by the kidney and result in rapid intravascular elimination of hemoglobin solutions, with such solutions having a 2-4 hour plasma half-life. Accordingly, efforts have been directed to overcome the inherent limitations of hemoglobin solutions by molecular modification of the hemoglobin. The purpose of the molecular modification is to stabilize hemoglobin to prevent dimer formation and to maintain the tense conformational state. Bunn et al., supra, demonstrated that cross-linking hemoglobin reduced renal elimination and increased intravascular retention time. Bunn et al. utilized bis (N-maleimidomethyl) ether; however, the resulting hemoglobin solution had a high oxygen affinity, i.e., a P.sub.50 of 3 mm Hg. Pyridoxal-5-phosphate has been demonstrated to have an analogous effect to 2,3-DPG in lowering oxygen affinity, resulting in a P.sub.50 of 26-30 mm Hg (Benesch, R. E., Biochem. 11: 2568 (1972)). However, unlike 2,3-DPG, pyridoxal phosphate does not act as a cross-linking agent, resulting in intravascular retention times similar to that of unmodified hemoglobin (Greenburg, A. G. et al., Surgery 86: 13 (1979)). Thus it was thought that pyridoxylation and cross-linking would be required to produce a blood substitute having low oxygen affinities (P.sub.50 equal to 20-30 mm Hg) and adequate intravascular retention times (half disappearance times of 20 or more hours).
In 1985, the Congress of the United States, Office of Technology Assessment (OTA), issued a report entitled "Blood Policy and Technology." At chapter 6 of this report, alternative sources of blood products were discussed, with the conclusion that the impetus to develop alternative blood sources and substitutes based on economic, safety, and availability considerations was a necessity. According to the report, the ideal red blood cell substitute would have six properties: 1) an oxygen dissociation curve and oxygen-carrying capacity similar to that of intact red blood cells; 2) be non-toxic and non-antigenic; 3) have good flow characteristics; 4) remain in the circulation for a long period of time; 5) have a long shelf life; and 6) be cost effective in comparison to present red blood cell transfusions. The report also concluded that no substitute yet developed fulfills all these criteria.
Four basic approaches have been utilized to develop an adequate blood substitute. In one approach, a class of synthetic compounds called perfluoro chemicals are being developed. In a second approach, synthesized analogues of hemoglobin are being developed. Investigators are also attempting to assemble a red cell by encapsulating hemoglobin in lipid vesicles called liposomes. Finally, purified hemoglobin has been chemically modified to prolong its circulation and enhance its oxygen binding-dissociation properties.
According to the OTA report, supra, to date, none of these approaches has proven satisfactory. The fluorocarbons are removed by the circulatory system as foreign substances, and they become lodged in the liver, spleen, and other tissues. Artificial cells made of membrane encapsulated hemoglobin have not been used for many reasons. The use of microcapsules made from synthetic polymers such as polystyrene, ethylcellulose, and silicone rubber introduces biologically incompatable materials into a living system. The cell walls of the capsules tend to leak, it is difficult to control permeability of the wall, and these capsules are too rigid and too large to pass through the capillary bed.
The use of blood and blood fractions is fraught with disadvantages. For example, the use of whole blood often is accompanied by the risk of transmission of hepatitis-producing virus and AIDS-producing virus which complicate the patient's recovery in the first instance and is fatal in the second. Additionally, the use of whole blood requires blood-typing and cross-matching to avoid immunohematological problems and interdonor incompatibility.
The blood fraction plasma (BFP) which is a physiologically balanced colloidal solution that fulfills many of the requirements of a blood volume expander, cannot be safely used for this purpose. The high incidence and the risk of transmitting homologous serum hepatitis associated with plasma is so great, that its use is no longer warranted.
The blood component hemoglobin possesses osmotic activity and the ability to transport and exchange oxygen, but it has the disadvantage of rapid elimination from circulation by the renal route and through vascular walls, resulting in a very short, and therefore, unsatisfactory half-life.
The literature, both patent and non-patent, is replete with efforts to produce a satisfactory blood substitute from polymerized, cross-linked, stromal free hemoglobin. Bonsen et al., U.S. Pat. No. 4,001,200, and Bonsen et al., U.S. Pat. No. 4,001,401 disclose polymerized, cross-linked, "stormal-free" hemoglobin and pharmaceutical compositions (and methods for using same) comprising the polymerized, cross-linked, "stromal-free" hemoglobin. The process for producing the polymerized, cross-linked, "stromal-free" hemoglobin of Bonsen et al. comprises lysing red blood cells, filtering through diatomaceous earth to remove stroma, dialyzing to remove residual low molecular weight salts and metabolytes, polymerizing to form water soluble, cross-linked, macromolecular, stromal-free hemoglobin, with a final sterilization by filtering through a filter having a pore size of about 0.20 to 0.45 microns. Included among the cross-linking agents disclosed by Bonsen et al. are dialdehydes such as glyoxal, malonic dialdehyde, succinic dialdehyde, glutaraldehyde, adipaldehyde, 3-methyl glutaraldehyde, propyladipaldehyde, phthalic dialdehyde, terephthaldehyde and malonic dialdehyde.
Bonsen et al. (III, U.S. Pat. No. 4,053,590), extends the disclosure of Bonsen et al. ('200) and Bonsen et al. ('401) with a discussion of physiologically acceptable polymeric plasma substitutes as carriers for the blood substitute. Further, applications for use as an artificial oxygen exchange solution in conventional oxygenators such as cardiac by-pass, extracorporeal circulatory assist devices, and hollow-fiber and sheet type membrane devices for use in assisting the circulation in ill patients, is suggested. Additionally, the polyhemoglobin is suggested as a source of protein and oxygen in the microbiological assay of foods for aerobic bacillus and staphyllococcus to ensure the food is safe for animal and human consumption and as a storing and preserving solution for viable isolated perfused mammalian organs for their eventual transplant into a recipient.
Bonhard et al., U.S. Pat. No. 4,136,093 discloses a hemoglobin preparation suitable for intravenous injection comprising a substantially pyrogen-free condensation product of hemoglobin and pyridoxal phosphate. The hemoglobin preparation is claimed to have a retention time in the blood system of from 2 to 9 hours. The product is produced by washing red blood cells with a weakly alkaline solution, hemolyzing, and treating the resulting material with a cation exchange resin. The material is separated from the resin, diluted to a hemoglobin concentration of about 5-9%, adjusted to a pH of about 7 to 9, treated with pyridoxal-5-phosphate and, optionally, treated with a solution of a borohydride and then a dialdehyde to cross-link the hemoglobin molecules. The non-pyrogenic nature of the infusion solution is obtained by, as a minimu, repeated washings with the weakly alkaline solution.
In Bonhard et al., U.S. Pat. No. 4,336,248, hemoglobin molecules were coupled to increase their intravascular residence time without significantly diminishing the oxygen transport ability of the molecule. The hemoglobin molecules are coupled to one another and/or to serum proteins and gelatin derivatives using dialdehydes such as aliphatic dialdehydes of 3-8 carbon atoms. Optionally, pyridoxal phosphate may be added subsequently. The coupled hemoglobin molecules are recovered by ammonium sulphate precipitation.
In Simmonds et al., U.S. Pat. No. 4,401,652, there is disclosed a process for preparing a "stromal-free" hemoglobin solution. The Simmonds et al. process is particularly adapted for large scale production of "stromal-free" hemoglobin, with reduced methemoglobin formation. The process comprises washing blood cells to remove non-cellular components, removing leukocytes, typically by filtration through a suitable adsorbent which preferentially retains the leukocytes, lysing the remaining red blood cells ultrasonically or mechanically, precipitation of the hemoglobin by mixture with a polyvalent cation, a polysulphate, and a polyvalent anion, and final purification by filtration and dialysis. The resulting hemoglobin solution is "substantially pure" and "free of stroma" and other lipoprotein cellular constituents and contains less than 5% methemoglobin.
Tye, U.S. Pat. No. 4,529,719, discloses "stromal-free" tetrameric hemoglobin which is cross-linked with certain bis-disalicyl esters and modified with pyridoxyl-5'-phosphate followed by reduction to produce bis-diamide covalently cross-linked, pyridoxyl-5'-phosphate covalently modified, tetrameric hemoglobin. The modified cross-linked "stromal-free" tetrameric hemoglobin is disclosed to be disease-free and capable of transporting oxygen to perfused tissue and remains in the intravascular space. Additionally, the product is suggested to be free from cell surface antigens, making it suitable for transfusion in place of red blood cells.
The modified cross-linked, "stromal-free" tetrameric hemoglobin of Tye is produced by starting with red blood cells of freshly drawn, outdated, or frozen packed cells or whole blood. The blood is drawn in sterile fashion into containers with sufficient anticoagulant activity to prevent clot formation. Hemoglobin from a variety of mammalian sources, such as human, bovine, ovine, or porcine are disclosed to be useful. Any non-heme protein is removed, preferably by zinc precipitation. Hemoglobin is released from the red blood cells by hypotonic lysis followed by ultrafiltration. The filtered hemoglobin is passed through a subsequent filtration step to remove virus particles, protein aggregates, and stromal elements. The typical filter has a nominal pore size of 0.020 microns and an exclusion for globular proteins of 1,000,000 Daltons. Zinc iron is added to precipitate the hemoglobin and the precipitate concentrated by filtration. The non-heme protein is removed in the filtrate. The resulting hemoglobin is then cross-linked using the bis-disalicyl esters and treated with pyridoxyl-5'-phosphate, followed by reduction of the reversible Schiff base covalent bond.
Kothe et al., U.S. Pat. No. 4,526,715 discloses a method for producing highly purified hemoglobin solutions free of plasma proteins and residual stromal lipids prepared from human blood or from animal blood in quantities large enough for clinical applications. The disclosed process comprises contacting red blood cells with a washing solution, hemolysing by introduction of the concentrated red blood cells into 2-3 times the volume of water, separating the stroma from the hemoglobin by ultrafiltration, and concentration in a third filtration stage utilizing a second ultrafiltration unit having a permeability of 10,000 to 50,000 Daltons.
However, in spite of the recent advances in the preparation of "stromal-free," cross-linked hemoglobin origin blood substitutes, the need has continued to exist for a blood substitute which is substantially free of endotoxins, phospholipids, and non-hemoglobin proteins, which is capable of 1) transporting adequate amounts of oxygen to tissue under ambient conditions; 2) having an oncotic activity equivalent to that of whole blood; 3) having an adequate intravascular retention time; 4) transfusible to all recipients without cross-matching or sensitivity testing; 5) free from disease agents such as bacteria and virus particles (hepatitis, AIDS, etc.); and 6) storable with minimum amounts of refrigeration.